Recently, industrial robots are required to make more flexible movement to copy with more complex jobs, and tend to have an increased number of control axes that can be controlled simultaneously. Generally, the necessary and sufficient conditions that a robot has to meet in order to reach a desired wrist position and a desired wrist posture are that the robot have six axes. However, recent applications demand for industrial robots having redundancy which have in excess of six axes.
In order for a robot wrist to make interpolated motion, the position and posture of the robot wrist on a certain path have to be controlled by the rotational angles of the respective articulations of the robot. However, since the robots having redundancy which have in excess of six axes require an enormous amount of calculations, it is difficult to process data on a real-time basis during robot operation, and such robots have not been put to practical use.
As one solution to the above problems, there has heretofore been proposed a control method for determining the rotational angle of a fourth axis so that the sum of or the difference between the cosine of the rotational angle of a second axis and the cosine of the rotational angle of a sixth axis will take an extreme value in a seven-axis robot, as disclosed in Japanese Patent Laid-Open No. 64-42704 (or No. 42704/89). Japanese Patent Laid-Open No. 64-16389 (or No. 16389/89) discloses a method of controlling the position and posture of a robot wrist through the reconstruction of a virtual six-axis robot with redundancy axes fixed.
The conventional control methods are, however, limited to a seven-axis robot or require a robot to be controlled as a virtual six-axis robot. As the redundancy axes are temporarily stopped, not all axes are controlled simultaneously, and the robot is not controlled for flexible movement.